A FT process comprises the hydrogenation of CO in the presence of a catalyst based on metals, such as Fe, Co and Ru. The products formed from this reaction are usually gaseous, liquid and waxy hydrocarbons which may be saturated or unsaturated. Oxygenates of the hydrocarbons such as alcohols, acids, ketones and aldehydes are also formed. The carbon number distribution of the products follow the well-known Anderson-Schulz-Flory distribution.
A heterogeneous Fisher-Tropsch process may be conveniently categorised as either a high temperature Fischer-Tropsch (HTFT) process or a low temperature Fischer-Tropsch (LTFT) process. The HTFT process can be described as a two phase Fischer-Tropsch process. It is usually carried out at a temperature from 250° C. to 400° C. and the catalyst employed is usually an iron-based catalyst.
The LTFT process can be described as a three phase Fischer-Tropsch process. It is usually carried out at a temperature from 220° C. to 310° C. and the catalyst employed is usually either a Co-based catalyst or a Fe-based catalyst. The conditions under which this process is carried out, results in the products being in a liquid and possibly also in a gas phase in the reactor. Therefore this process can be described as a three phase process, where the reactants are in the gas phase, at least some of the products are in the liquid phase, and the catalyst is in a solid phase in the reaction zone. Generally this process is commercially carried out in a fixed or fluidized bed reactor or a slurry bed reactor.
During FT synthesis (FTS) another reaction, namely the water gas shift (WGS) reaction usually also takes place. The WGS reaction is as follows:H2O+COCO2+H2 
The WGS reaction is not a desired reaction in FTS where the H2/CO molar feed ratio is high, that is above the stoichiometric ratio required for the products to be formed. This is due to the fact that during the WGS reaction CO is converted to unwanted CO2 compared to FTS where CO is converted to hydrocarbons. It will also be appreciated that CO2 production creates environmental problems.
The WGS reaction is especially problematic in a LTFT process in the presence of a Fe-based catalyst, as these reactions are usually carried out between 220 to 270° C. Under these conditions the WGS reaction is not under equilibrium and no effective reverse WGS takes place with the result that CO2 is not converted back to CO within the LTFT reactor.
It has been observed that during FTS, the WGS activity of a FT catalyst increases over time and an increase in CO2 selectivity is accordingly also observed.
It has also been observed now, that with an increase in WGS activity (and accordingly CO2 production) acid production increases. Furthermore, it has been also observed that with increased acid production a lesser degree of olefin isomerisation (internal double bonds/total double bonds) takes place.
In the light of the above it was expected that the addition of an acid to FTS would increase CO2 production and reduce olefin isomerisation. However, it was most surprisingly found that the addition of an acid to FTS in fact resulted in a decrease in CO2 selectivity, a decrease in acid selectivity (in at least some cases) and in at least some cases little or no change in olefin isomerisation.
The applicant is not aware of any prior art wherein an acid had been introduced in FTS. This is not surprising as the production of acids via FTS is a major problem. The acids may cause corrosion of mild steel and may cause deactivation and corrosion of hydrotreating catalysts in the downstream refinery. Carboxylates may also cause bed plugging on hydrotreating catalysts. In addition, strict specifications on the acid content of commercial fuels exist.